Question Video: Determining Which Liquid Will Have the Greatest Vapor Pressure given the Structural Formula Chemistry

Video Transcript

Which of the following liquids have the greatest vapor pressure at 25 degrees Celsius?

Five displayed formulas are shown. In this question, we are given the displayed formulas of the molecules of five different liquids. And we are being asked to determine which of these liquids will have the greatest vapor pressure at a temperature of 25 degrees Celsius.

The vapor pressure of a liquid is the equilibrium pressure exerted by a vapor on the surface of the liquid phase. The vapor above the surface of the liquid is produced by the process of evaporation. The strength of the intermolecular forces between molecules impacts how easily a liquid evaporates. Intermolecular forces are defined as the forces of attraction or repulsion that act between neighboring particles, such as atoms, ions, or molecules.

In this video, we will put our focus on the forces of attraction between molecules. To determine which of the liquids would have the greatest vapor pressure at 25 degrees Celsius, we will need to closely examine each structure provided in the answer choices to identify and compare the strengths of the intermolecular forces.

There are three types of intermolecular forces which can be present between molecules: hydrogen bonds, dipole-dipole interactions, and London dispersion forces. Hydrogen bonds form between a hydrogen atom covalently bonded to a fluorine, oxygen, or nitrogen atom and a lone pair of electrons on a fluorine, oxygen, or nitrogen atom of a nearby molecule. Let’s begin examining the molecules in the answer choices to find out if hydrogen bonding could be present.

The molecular structure shown in answer choice (A) contains two oxygen-to-hydrogen single bonds. And even though it’s not shown here, the oxygen atoms each have two lone pairs of electrons. Therefore, hydrogen bonds will form between the molecules in this liquid. The molecular structure shown in answer choice (B) contains a total of three polar bonds. One of these polar bonds is an oxygen-to-hydrogen single bond. The presence of this bond indicates that hydrogen bonding will be present between the molecules in this liquid.

When looking at the displayed formula in answer choice (C), we see that this molecule also has one oxygen-to-hydrogen single bond. Therefore, like liquids (A) and (B), hydrogen bonds will be present between the molecules in this liquid. The molecular structure in answer choice (D) contains one polar bond, a carbon-to-oxygen double bond. This bond is not composed of a hydrogen atom covalently bonded to a fluorine, oxygen, or nitrogen atom. So hydrogen bonds cannot form between the molecules of this liquid.

Let’s draw a more simplified condensed structural formula for this molecule. This molecule has a bent molecular shape and is nonsymmetrical. The bond dipole of the carbon-to-oxygen double bond is not counteracted by any other polar bonds in the molecule. Therefore, this molecule is polar and has a molecular dipole oriented toward the oxygen atom.

Dipole-dipole interactions are a type of intermolecular force present between polar molecules that cannot hydrogen bond. Dipole-dipole interactions will occur between neighboring molecules in liquid (D) due to the partial positive charge on the carbon atom and the partial negative charge on the oxygen atom in the carbon-to-oxygen double bond.

Finally, the displayed formula shown in answer choice (E) is similar to that shown in answer choice (D). The molecular structure contains one polar bond, a carbon-to-oxygen double bond. If we drew a condensed formula, we’d see that this molecule also has a bent nonsymmetrical shape. Therefore, the molecules of this liquid are polar. Due to the partial positive and negative charges on the carbon and oxygen atoms in the double bond, dipole-dipole interactions will be present between the molecules of this liquid.

When comparing the relative strengths of the three types of intermolecular forces, hydrogen bonds are the strongest. The stronger the intermolecular forces between molecules, the lower the vapor pressure. This is because more energy is required to disrupt the strong attractions between liquid molecules so that they can escape the liquid and become vapor. Because hydrogen bonds are stronger intermolecular attractions than dipole-dipole interactions, liquids (A), (B), and (C) will have lower vapor pressure than liquids (D) and (E). Therefore, we can eliminate answer choices (A), (B), and (C) and put our focus on comparing answer choices (D) and (E).

London dispersion forces are present between molecules with temporarily induced dipoles. They are the only type of intermolecular force possible between nonpolar molecules. However, they are present between the molecules of all liquids. The strength of London dispersion forces depends on the molecular mass of the molecules. The molecules in liquid (E) have a smaller molecular mass than the molecules in liquid (D). Therefore, the London dispersion forces between molecules in liquid (E) will be weaker than those in liquid (D).

Because the intermolecular forces between the molecules in liquid (E) are the weakest when compared to all the other answer choices, liquid (E) would have the greatest vapor pressure. Of the five liquids provided in this problem, liquid (E) would have the greatest vapor pressure at 25 degrees Celsius.